At present there is a great need for a method of producing hard copies of electronically stored information using electronically modulated exposure sources such as lasers, laser diodes and light emitting diodes. As many of these devices produce radiation in the infrared region of the electromagnetic spectrum, infrared sensitive recording materials are required for imaging.
U.S. Pat. Nos. 2,955,939 and 4,581,325 disclose the use of meso-substituted cyanine dyes incorporating a barbituric acid substituent and a five membered bridging moiety on the methine chain, as spectral sensitisers.
U.S. Pat. Nos. 3,552,974 and 3,623,881 disclose the use of meso-amino substituted bridged chain heptamethine cyanine dyes as spectral sensitisers.
U.S. Pat. No. 4,153,463 and British Patent No. 1565593 disclose the use of meso-substituted trimethine cyanine dyes incorporating a nitroalkyl group as thermally bleachable antihalation dyes in Dry Silver materials. However, the dyes are not known as spectral sensitisers and appear to desensitise the material.
U.S. Pat. No. 4,283,487 and British Patent No. 1058116 disclose the use of merocyanine dyes incorporating a nitro-alkane end group as thermally bleached acutance dyes in Dry Silver materials.
European Patent No 288261 discloses a number of examples of cyanine dyes for use as sensitisers in the infrared region of the spectrum. In particular a cyanine dye is disclosed comprising a heptamethine chain bridged across its three central carbon atoms by a saturated fragment which completes a five or six membered ring. Furthermore the central atom of the heptamethine chain may have a substituent such as a halogen atom, substituted amino group or a ketomethylene moiety of the type known to the art of merocyanine and oxonol dye synthesis.
The prior art discloses the use of meso-substituted trimethine cyanine dyes incorporating nitroalkyl groups and merocyanine dyes incorporating nitroalkyl end groups for use as thermally bleachable anti-halation dyes in photothermographic material. However, such dyes are not known as spectral sensitisers in conventional silver halide photographic materials.
U.S. Pat. No 2,955,939 discloses the electron withdrawing meso-substituents on heptamethine cyanine dyes such as chloro, amino, barbituric acid and dicyanomethylene groups and they are stated to be good spectral sensitisers. However, cyanine dyes for use in spectral sensitisation of photographic materials almost invariably do not contain nitro substituents and generally their incorporation into a cyanine dye molecule, especially as a ring substituent, causes a decrease in the spectral sensitising ability of the dye and in some cases produces a desensitising dye, e.g., PINAKRYPTOL YELLOW.